Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide a system and method for protecting one or more circuit components. Merely by way of example, some embodiments of the invention have been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
Schottky rectifying diodes with low forward voltages are often used in power conversion systems to improve system efficiency. Generally, a conventional power conversion system often uses a transformer to isolate the input voltage on the primary side and the output voltage on the secondary side. To regulate the output voltage, certain components, such as TL431 and an opto-coupler, can be used to transmit a feedback signal from the secondary side to a controller chip on the primary side. Alternatively, the output voltage on the secondary side can be imaged to the primary side, so the output voltage is controlled by directly adjusting some parameters on the primary side. Then, some components, such as TL431 and an opto-coupler, can be omitted to reduce the system costs.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system with primary-side sensing and regulation. The power conversion system 100 includes a primary winding 110, a secondary winding 112, an auxiliary winding 114, a power switch 120, a current sensing resistor 130, an equivalent resistor 140 for an output cable, resistors 150 and 152, and a Schottky rectifying diode 160. For example, the power switch 120 is a bipolar junction transistor. In another example, the power switch 120 is a MOS transistor.
To regulate the output voltage within a predetermined range, information related to the output voltage and the output loading often needs to be extracted. For example, when the power conversion system 100 operates in a discontinuous conduction mode (DCM), such information can be extracted through the auxiliary winding 114. When the power switch 120 is turned on, the energy is stored in the secondary winding 112. Then, when the power switch 120 is turned off, the stored energy is released to the output terminal during a demagnetization process. The voltage of the auxiliary winding 114 maps the output voltage on the secondary side as shown below.
                              V          FB                =                                                            R                2                                                              R                  1                                +                                  R                  2                                                      ×                          V              aux                                =                      k            ×            n            ×                          (                                                V                  0                                +                                  V                  F                                +                                                      I                    0                                    ×                                      R                    eq                                                              )                                                          (                  Equation          ⁢                                          ⁢          1                )            where VFB represents a voltage at a node 154, and Vaux represents the voltage of the auxiliary winding 114. R1 and R2 represent the resistance values of the resistors 150 and 152 respectively. Additionally, n represents a turns ratio between the auxiliary winding 114 and the secondary winding 112. Specifically, n is equal to the number of turns of the auxiliary winding 114 divided by the number of turns of the secondary winding 112. Vo and Io represent the output voltage and the output current respectively. Moreover, VF represents the forward voltage of the rectifying diode 160, and Req represents the resistance value of the equivalent resistor 140. Also, k represents a feedback coefficient as shown below:
                    k        =                              R            2                                              R              1                        +                          R              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
FIG. 2 is a simplified diagram showing a conventional operation mechanism for the flyback power conversion system 100. As shown in FIG. 2, the controller chip of the conversion system 100 uses a sample-and-hold mechanism. When the demagnetization process on the secondary side is almost completed and the current Isec of the secondary winding 112 almost becomes zero, the voltage Vaux of the auxiliary winding 114 is sampled at, for example, point A of FIG. 2. The sampled voltage value is usually held until the next voltage sampling is performed. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref. Therefore,VFB=Vref  (Equation 3)
Combining Equations 1 and 3, the following can be obtained:
                              V          0                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              0                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          4                )            Based on Equation 4, the output voltage decreases with the increasing output current.
But thermal runaway may occur in the Schottky diode 160 if the temperature of the diode 160 exceeds a threshold, and a reverse leakage current increases in magnitude drastically. If the output load of the power conversion system 100 is reduced, the reverse leakage current continues to increase in magnitude and the temperature of the diode 160 does not decrease. As such, once the thermal runaway occurs in the Schottky diode 160, the temperature of the diode 160 keeps higher than a normal operating temperature even if the output load is reduced, which may cause safety problems. For example, the outer shell of the power conversion system 100 may be melted due to the high temperature of the Schottky diode 160.
Hence it is highly desirable to improve the techniques of system protection.